Pollution in thunderclouds increases global warming

Kate Taylor, 21st May 2012

Pollution is leading thunderstorm clouds to capture heat, increasing global warming in a way that climate models have failed to take into account.

It strengthens them, causing their anvil-shaped tops to spread out high in the atmosphere and capture heat, especially at night, says Jiwen Fan of the Department of Energy's Pacific Northwest National Laboratory.

"Global climate models don't see this effect because thunderstorm clouds simulated in those models do not include enough detail," says Fan. "The large amount of heat trapped by the pollution-enhanced clouds could potentially impact regional circulation and modify weather systems."

Thunderstorm clouds - known as deep convective clouds - are an important part of the climate cycle. They reflect a lot of the sun's energy back into space, trap heat that rises from the surface and return evaporated water back to the surface as rain.

Previous work has shown that when it's not too windy, pollution leads to bigger clouds. This occurs because more pollution particles divide up the available water for droplets, leading to a higher number of smaller droplets that are too small to rain. Instead, they ride the updrafts higher, where they freeze and absorb more water vapor. Collectively, these events lead to bigger, more vigorous convective clouds that live longer.

To find out which factors contribute the most to this invigoration effect, the team set up computer simulations for two different types of storm systems: warm summer thunderstorms in southeastern China and cool, windy frontal systems on the Great Plains of Oklahoma.

The simulations had a resolution that was high enough to allow the team to see the clouds develop. The researchers then varied conditions such as wind speed and air pollution.

And they found that for the warm summer thunderstorms, pollution led to stronger storms with larger anvils. Compared to cloud anvils that develop in clean air, the larger anvils both warm more, by trapping more heat, and cool more, by reflecting additional sunlight back to space. On average, however, the warming effect dominates.

The warming was surprisingly strong at the top of the atmosphere during the day when the storms occurred. The pollution-enhanced anvils also trapped more heat at night, leading to warmer nights.

"Those numbers for the warming are very big, but they are calculated only for the exact day when the thunderstorms occur," says Fan. "Over a longer time-scale such as a month or a season, the average amount of warming would be less because those clouds would not appear everyday."